The present invention relates to an optical fiber coupler having light-coupling, light-branching and light-coupling-branching abilities, in particular, an optical fiber coupler of a wide wavelength range whose light-branching ratio shows low wavelength-dependency, as well as a method for preparing the optical fiber coupler.
The optical fiber coupler is a device incorporated into an optical fiber communication system and serves to couple or branch light signals or serves to couple or branch light signals having different wavelengths. In the field of the optical fiber communication system, in particular, such a system which makes use of the so-called wavelength division multiplex-transmission system in which a plurality of optical signals having different wavelengths are transmitted through a single transmission line, there has been desired for the development of a optical fiber coupler of a wide wavelength range (hereunder also referred to as "wide wavelength range-optical fiber coupler") whose light-branching ratio has low wavelength-dependency.
Examples of conventional wide wavelength range-optical fiber couplers of this type include those disclosed in Japanese Patent Provisional Publication No. 2-236507 which are produced by arranging parallel to each other two single mode optical fibers whose clads have different outer diameters, fusing these fibers by heating with an oxyhydrogen burner, orientating the fused portion of the fibers while weakening the flame and swinging the burner to thus form a light-coupling portion; and those disclosed in Japanese Patent Provisional Publication No. 2-156210 which are produced by fusing and orientating a plurality of optical fibers having different core diameters to form a light-coupling portion.
In the optical coupler as disclosed in Japanese Patent Provisional Publication No. 2-236507, one of the optical fibers used has a clad diameter (outer diameter of the clad layer) smaller than that of the other optical fiber and accordingly, the mechanical strength of the resulting coupler is insufficient and this makes the handling thereof difficult. Since two optical fibers arranged parallel to one another and having different outer diameters are simultaneously heated, the fiber having a smaller diameter is first softened and causes deformation. This, correspondingly, makes the formation of a well-shaped light-coupling portion difficult. Therefore, this technique does not ensure the production of a wide wavelength region-optical fiber coupler having excellent optical properties. Moreover, one lead portion of the resulting optical fiber coupler has an outer diameter different from that of the usual optical fiber. This makes the connection of the coupler with other optical fibers difficult and, in turn, leads to an increase in the connection loss.
On the other hand, the light-coupling portion formed from optical fibers having different core diameters as disclosed in Japanese Patent Provisional Publication No. 2-156210 likewise suffers from a problem in that an increase in the connection loss would be observed when the optical fiber coupler is connected to another optical fiber due to differences in the cut-off wavelength and in the mode field diameter between the optical fibers used as starting materials for the formation of the light-coupling portion.